A technique of estimating an internal state of a rechargeable battery by performing regression analysis on a charging curve of the rechargeable battery has conventionally been proposed. However, in such a regression analysis, it is difficult to estimate an internal state of the rechargeable battery with high accuracy, especially when charging the rechargeable battery at a high rate, without considering any influence of a diffusion resistance of an active material in a case where the diffusion resistance is small. On the other hand, if the influence of the diffusion resistance of the active material is taken into consideration when estimating the internal state of the rechargeable battery, an amount of calculation and working area (a storage area temporarily used for data processing) required would be increased.